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Old February 12th, 2004, 09:20 AM   #1
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Ieee Fp

Can someone explain bit by bit the difference between a 16 bit register and a 32 bit floating point register and how the FP is
read in an HMI. Beside the fact that the FP uses two registers and can hold a larger value.
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Old February 12th, 2004, 09:38 AM   #2
Ron Beaufort
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maybe this will get you started ...

I’m assuming (gosh I hate that word) that you’re using an Allen-Bradley system ...

this is the best explanation I’ve ever seen ...

SLC Instruction Set Reference Manual

take a look at page F-6 [Adobe Reader page 608 of 648] ...

the individual bits are not normally viewable ... but ... if you want to examine the bits in the floating point location ... try using the COP instruction to copy the floating point value into two consecutive integer words ... it’s been quite awhile since I tried this but if I remember correctly, it will reveal the bit pattern for you ...

Peter Nachtwey is one of the best resources around for topics like this ... maybe he’ll drop by and answer any specific questions that you might have ... if you want “how to” help on your HMI, etc. then you’ll get better responses if you’ll post again and tell us what system you’re using ...

2-B ?
Best regards, ----+----] [----+------------( )----
Ron | |
PLC Training Boot Camp - Retired | 2-B |

I once was lost, but now am found, was blind, but now I see.

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Old February 12th, 2004, 09:48 AM   #3
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This one is a modicon. but I do have some AB's TI's Mitchi*****i,
Triplex, Triconex, Etc. The problem I have is in this particular
instance is an analog input that is converted to a FP and then is compared with a setpoint in another FP register. I'm not sure what the value of the setpoint should be. I like to research things myself but am having a little trouble with finding a satisfactory answer.
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Old February 12th, 2004, 09:58 AM   #4
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The layout example Ron pointed to is pretty good. It's a bit thin of how to actually determine a value (as I remember the mantissa has an assumed base) but it does give you a feel for the bit pattern.
One thing to be a little careluf of is that what some HMIs refer to as a 'floating point' number is actually a formatted integer. For example, the HMI might let you enter a floating point representation as a double integer but will send that back as the double integer with the value plus a byte or single integer with a decimal point location. This is not the format of a float in the SLC. If you send a float out from an SLC to an HMI that uses formatted double integers the number will look REALLY screwy.

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Old February 12th, 2004, 10:14 AM   #5

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I've been investigating a little in the nature of floating point regs in Allen Bradley SLC's as I was trying to convert a long integer (32-bit integer) into a floating point and back. I needed accurate numbers up to +/- 2 billion for a positioning system.

The SLC does not support long integers, so I was trying to use floating point. I works well until the floating point register have to convet the value into 1.xxxxxx powered by a number of tens (e.g. 1.23456+E008). When the floting point reaches this number, the rounding is a problem (to my situation, as could not accept to miss out up to approx. 25 numbers when the value closed in on +/- 2 billion.

I even went to speak with Allen Bradley to find a way to extract a floating point value into a double integer (as the SLC do not support long integers), but the answer was, that they would ask me to pay development time for them to create the nescessary code/instruction set. This was back in january 2004.
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Old February 12th, 2004, 10:26 AM   #6
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You evidently do not have nights and weekends comprehensive support
with AB. I to have had problems with Rockwell support. Even when it was a problem with "their" code.
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